Don't allow OS::SNPrint or OS::VSNPrint to return negative values.

vm/os_win.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/os.h"
 
 #include <time.h>
 
 #include "platform/assert.h"
 
@@ skipping 137 matching lines @@
   va_list args;
   va_start(args, format);
   int retval = VSNPrint(str, size, format, args);
   va_end(args);
   return retval;
 }
 
 
 int OS::VSNPrint(char* str, size_t size, const char* format, va_list args) {
   if (str == NULL || size == 0) {
-    return _vscprintf(format, args);
+    int retval = _vscprintf(format, args);
+    if (retval < 0) {
+      FATAL1("Fatal error in OS::VSNPrint with format '%s'", format);
+    }
+    return retval;
   }
   va_list args_copy;
   va_copy(args_copy, args);
   int written =_vsnprintf(str, size, format, args_copy);
   va_end(args_copy);
   if (written < 0) {
     // _vsnprintf returns -1 if the number of characters to be written is
     // larger than 'size', so we call _vscprintf which returns the number
     // of characters that would have been written.
     va_list args_retry;
     va_copy(args_retry, args);
     written = _vscprintf(format, args_retry);
+    if (written < 0) {
+      FATAL1("Fatal error in OS::VSNPrint with format '%s'", format);
+    }
     va_end(args_retry);
   }
   // Make sure to zero-terminate the string if the output was
   // truncated or if there was an error.
   if (written >= size) {
     str[size - 1] = '\0';
   }
   return written;
 }
 
@@ skipping 25 matching lines @@
 void OS::Abort() {
   abort();
 }
 
 
 void OS::Exit(int code) {
   exit(code);
 }
 
 }  // namespace dart